The objective of the research is to study the occurrence of twins in the offspring of twins themselves, in the offspring of sibs of twins, and in the grandchildren of the twins and sibs. By computing twinning rates for these various groups we propose to investigate the method of inheritance of twinning. Our results so far have indicated that dizygotic twinning is inherited and that the mode of inheritance may be maternal recessive with incomplete penetrance. Preliminary results, based on data for the grandchildren of unlike sexed twins and their sibs, indicate an apparent inability of males to transmit the twinning factor to their daughters, a finding which is so far unexplained in terms of nuclear inheritance. We hope to carry these investigations further by continuing to collect and analyze data for the grandchildren of twins and their sibs. In addition, we shall study the fertility and longevity of twins, sibs of twins and parents of twins. Results to date, which are for completed families, show that twins have a lower fertility than their sibs and a shorter life span. Our data are from the more than four million family record sheets contained in the Archives of the Genealogical Society of the Mormon Church. We have made a systematic search of these records for families containing at least one set of twins and data relating birth, death, marriage and offspring.